1. Field of the Invention
This invention relates to an information processing device which has an information embedding function to embed predetermined information in an image and an embedded information extract function to extract the embedded information from the image.
2. Description of the Related Art
With development of the technology of data communication networks, such as the Internet, a technique to protect multimedia data of images, video, digital audio, etc. over data communication networks from unauthorized uses attracts attention.
Recently, research of the technique for embedding predetermined information in multimedia data by using steganography, digital watermarking, etc. is advanced for the purposes of copyright protection, data tracing, falsified data detection, meta information addition, etc. Digital watermarking is a technique to embed data in digital audio, images or video without affecting the image quality or sound quality. Steganography is a technique to write hidden messages in such a way that no one apart from the sender and intended recipient even realizes there is a hidden message. Steganography includes the concealment of digital information within computer files.
In such information embedding technique, it is important to extract the embedded information from the image data with sufficient accuracy after the predetermined information is embedded in the image data.
For example, as disclosed in Japanese Laid-Open Patent Applications No. 2000-216983, No. 2001-333267 and No. 2006-135922, a method of embedding information to a number of image regions of image data repeatedly is proposed in order to provide a method of extracting the embedded information with sufficient accuracy. Once the information is embedded in an image repeatedly, the information can be extracted from the whole image repeatedly and the extracted information items can be unified, so that the possibility that the original information is extracted correctly becomes high even when the information is partially lost or an error arises.
However, the information embedding method as disclosed in Japanese Laid-Open Patent Applications No. 2000-216983, No. 2001-333267 and No. 2006-135922 has a problem in that, although information may be easily embedded in an image region with a large optical density within an image, it is impossible to embed information in an image region with a small optical density within the image.
In a case of an image in which the applicability of information embedding varies depending on the optical density of the image (or wherein the applicable image region to which information embedding can be applied and the non-applicable image region to which information embedding cannot be applied coexist), if the repeatability cycle (alignment of the information being embedded) and the applicability cycle (alignment of the applicable image region and the non-applicable image region) are in agreement, an appearance ratio of particular parts of the embedded information (partial information items) when extracting the embedded information may decrease greatly. In such a case, a problem arises in that the extraction accuracy of the partial information items falls.
The falling of the extraction accuracy mentioned above becomes remarkable in a case of a document image in which characters, photographs, figures, etc. coexist. Generally, a document image contains many white regions, and it is difficult to embed invisible information in a white region. If particular partial information items of the invisible information are assigned to the white regions of a document image repeatedly, the repeatability cycle of the partial information items will decrease and the extraction accuracy of the partial information items will fall.
For this reason, when the image regions in which information can be embedded are restricted, it is difficult to raise the extraction accuracy of the extracted information simply by assigning information items to the image regions repeatedly and embedding the information using the information embedding method according to the related art.